1. Field of the Invention
This invention relates to an original scanning apparatus for use in a copying machine or the like and, more particularly, to an original scanning apparatus for directing light from an original to a photoelectric conversion device or a photosensitive member by moving a plurality of mirrors parallel to the original surface.
2. Description of the Related Art
An apparatus such as the one illustrated in FIG. 7 is a conventional original scanning apparatus of this kind.
FIG. 7 is a side view of an original scanning apparatus using a photoelectric conversion device such as a charge coupled device (CCD). An original 105 is illuminated with an original illumination lamp 104. Reflected light from the original 105 is sent to a photoelectric conversion element unit 107 by a first reflecting mirror 101, a second reflecting mirror 102, a third reflecting mirror 103 and a reduction lens 106. The original illumination lamp 104 and the first reflecting mirror 101 are mounted on a first mirror base 108, which is driven with a wire 110 and a pulley 112 to scan the original at a constant speed to the rightward direction as viewed in FIG. 7. The second reflecting mirror 102 and the third reflecting mirror 103 are mounted on a second mirror base 109 through a supporting structure. The second mirror base 109 is driven with the wire 110 and the pulley 112 to move in the same direction as the original illumination lamp 104 and the first reflecting mirror 101 at half the speed of these components, whereby the optical distance between the original 105 and the photoelectric conversion element unit 107 can be constantly maintained.
For high-precision image formation using the above-described original scanning apparatus, it is necessary to always move the moving components at constant speeds through the original scanning period. In the case of an application to a copying machine or the like, the copying magnification in the direction in which original 105 is scanned is adjusted by selecting the scanning speed. A range of rotational speed of a driving motor 111 from a low speed to a high speed about ten times the low speed is required to select the scanning speed in accordance with the desired ratios of enlargement and reduction of original 105. In order to perform scanning immediately after a preceding cycle of scanning, it is necessary to return the original illumination lamp 104 and the first to third reflecting mirrors 101, 102, and 103 to the start position at speeds much higher than the speeds at the time of scanning. To achieve this, motor speed control over a wider range is required. If the apparatus is arranged so as to satisfy these conditions, vibration is transmitted to the reflecting mirrors from, for example, the rotational speed of the driving motor 111 during scanning, across a wide frequency range.
FIG. 8 shows the conventional reflecting mirror structure, particularly the second and third reflecting mirrors 102 and 103, the second mirror base 109 and the supporting structure. As shown in FIG. 8, the second and third mirrors 102 and 103 are formed of the same material, have the same shape and are supported at the same positions in the vicinity of their opposite longitudinal ends by a three-point supporting structure having supporting members 113.
FIG. 9(a) is a graph showing a frequency transmission characteristic of the structural body of the conventional original scanning apparatus, i.e., response of the second and third reflecting mirrors 102 and 303 in the scanning direction in the apparatus affected by a disturbance through the wire 110. As shown in FIG. 9(a), the second and third reflecting mirrors 102 and 103 have the same frequency transmission characteristic. A peak at a frequency of about 200 Hz corresponds to the first bending mode in the longitudinal direction of the mirrors, as shown in FIG. 9(b).
The mirrors 102 and 103 vibrate sympathetically if, during original scanning, the rotational speed of the driving motor 111 at a certain copying magnification becomes equal to a value corresponding to a frequency close to 200 Hz, i.e., the natural frequency of the first bending mode shown in FIG. 9(a). If the amount of deformation caused by this vibration becomes equal to or larger than a certain value, the error in the optical path of the reflected light beam exceeds the allowable range of an element of the photoelectric conversion element unit 107, resulting in a deterioration in image quality; i.e., a pixel misalignment.
In the conventional reflecting mirror structure described above, such a deterioration in image quality may occur because the amounts of deformations of the second and third reflecting mirrors 102 and 103 are maximized at the same frequency.